1. Field of the Invention
This invention relates to a catalytic structure for treating exhaust gases. More specifically it relates to a catalytic structure having a high catalyst-retainability and also a superior acid resistance.
2. Description of the Prior Art
Heretofore, packed columns containing a granular catalyst have been employed for exhaust gas treatment such as selective reduction of nitrogen oxides contained in exhaust gases, but such a problem has been raised that fine particles entrained in the gases to be treated, adhere onto and deposit on the catalyst layer to increase the pressure loss within the system, and in the case of treating a large amount of the gases, this has a direct influence upon the running cost. As for such a problem, even in the case of exhaust gases having a relatively low content of dusts, not to mention the case of dirty gases, the properties of the deposited dusts vary with the lapse of time, resulting in increasing the adherence of dusts. In order to avoid this, an apparatus for treating exhaust gases wherein a plate-shaped catalyst is arranged in parallel to the stream of gases to be treated, have come to be employed.
Catalytic structures employed in such an apparatus can be classified into two types i.e. a structure wherein a catalyst itself is shaped into a plate and a structure wherein a material other than catalyst is employed as a substrate on which a catalyst is supported. In the case of the former plate-shaped catalyst, a catalyst component of a relatively high cost is required in a large amount, whereas in the case of the latter, this drawback is overcome, resulting in a practical advantage. In the case of the latter, metallic materials having a superior workability are often chosen for the substrate. For example, a plain weave, square mesh, metal net disclosed in Japanese patent application laid-open No. 116,168/1976 and similar network substrates are mentioned.
Preferable conditions for the catalytic structure having a catalyst supported on a substrate are a high catalyst-retainability and resistance to corrosive environment brought about by exhaust gases.
As for the means for enhancing the catalyst-retainability, a method of providing projections on a metallic plate, a method of boring holes in various kinds of forms in a metallic plate, and the like methods may be devised, but in such cases where a catalytic substance is adhered onto these substrates, there is a drawback that the catalytic substance is liable to peel off through contact or vibration.
On the other hand, the corrosive environment is brought about mainly by sulfur trioxide (SO.sub.3) contained in exhaust gases, and the substrate for catalyst must endure such a corrosive environment, particularly a severe condition as in the case of corrosion in the vicinity of dew point of water vapor containing sulfuric acid. Such a corrosive environment cannot be avoided when gases to be treated are started or stopped to be passed. Further, iron compound such as Fe.sub.2 O.sub.3 formed by corrosion of the substrate for catalyst, etc. has a catalytic acitivity of oxidizing SO.sub.2 contained in exhaust gases into sulfur trioxide (SO.sub.3). Thus, the substrate for catalyst must have a sufficient corrosion resistance. Accordingly, a catalytic structure satisfying both the catalyst-retainability and the acid resistance at the same time has been desired.